Abstract: Objective To explore the exact mechanism of the vertical transmission of hepatitis B virus (HBV ).Methods Polymerese chain reaction technique was used to detect the HBV DNA in human early embryo(age< 7weeks);in which the serum hepatitis B surface antigen(HBsAg) was positive in mothers and negative in fathers.Serum HBV DNA of the mothers were detected by the same technique.Results HBV DNA was positive in 7 human early embryos(22.58%)of 31 cases,HBV DNA was positive in 5 early embryos,among 20 mothers whose serum hepatitis Be antigen(HBeAg) and/or HBV DNA was positive (25;00%).HBV DNA was detected in 2 early embryos,among 11 mothers whose serum HBeAg and HBV DNA were negative(18.18%).The positive rate of HBV DNA in the two groups was not significant(P>0.05 ).Conclusion The infection of HBV happened in the early embryo.It provided the possiblity of HBV infecting embryo via the germ line.This can evenly happened in those mothers without replicated markers of HBV in serum.